The invention relates to photonic crystals, and in particular to composite photonic crystals comprising one- and two-dimensionally periodic photonic crystals.
The development of photonic crystals, structures with band gaps that prevent the propagation of light in a certain frequency range, has led to proposals of many novel devices for important applications in lasers, opto-electronics, and communications. Among these devices are high-Q optical filters, waveguides permitting tight bends with low losses, channel-drop filters, efficient LEDs, and enhanced lasing cavities. All of these devices, however, require the fabrication of photonic crystals allowing confinement of light in three dimensions. Moreover, the length scale of the features in a structure must be on the order of microns in order to control light of wavelengths typical in opto-electronics and other applications.
It is therefore desirable to obtain quasi-3D photonic crystals that approximate the most desirable properties of true three-dimensionally periodic crystals, but are potentially much easier to fabricate since they are composed of lower-dimensionally periodic components. The most straightforward way of achieving a photonic crystal in three dimensions is to utilize a structure with a true three-dimensional band gap. Here, light whose frequency is in the band gap cannot propagate in any direction through the crystal. Such a structure has been described in U.S. Pat. Nos. 5,440,421 and 5,600,483, of common assignee, and much effort has been expended in attempting to construct it. Fabrication of this and other true 3D structures has proved difficult, however, due to the complicated three-dimensional connectivity of these crystals and their extreme sensitivity to inter-layer alignment.
An alternative approach that has been successfully applied is to take a one- or two-dimensionally periodic photonic crystal and to truncate it in the second and/or third dimensions using index confinement, classically known as total internal reflection (TIR), and described in U.S. Pat. No. 5,526,449. The foregoing patent refers to structures with one- or two-dimensional periodicities and band gaps as 1D or 2D photonic crystals, respectively, although they may exist in three dimensions. TIR allows light to be guided within a high index (high dielectric constant, .di-elect cons.) material surrounded by a low index material. When this effect is combined with a photonic crystal, the resulting hybrid can approximately localize light in a point-like region, a resonant cavity, where the structure has been altered. This alteration, and the resulting resonant cavity, is referred to as a point defect.
Light can even be perfectly guided along a waveguide or line defect, a one-dimensionally periodic line-like region of defects in the periodic structure. A one-dimensionally periodic TIR-based photonic crystal has, in fact, been fabricated successfully as reported by Foresi et al., "Measurement of Photonic Band Gap Waveguide Microcavities," Nature 390, p. 143-145 (1997). Since it does not have a true three-dimensional band-gap, however, light cannot be perfectly confined. Instead, it leaks out slowly. The lifetime according to which localized light leaks out is described by the quality factor, or Q, of the defect. For most applications (including all of those mentioned above), it is critical that the Q of a point defect be as large as possible. Although TIR designs provide adequate values of Q, one would like to do better.